Starship Asterisk*

The researchers compared the sizes of the craters to simulations say the atmospheric pressure of Mars 3.6 billion years ago would have been much greater than it is today.

However, that pressure is less than required to warm the surface above freezing.

Instead, short-lived surface warming by volcanic eruptions, asteroid impacts or orbital changes of the planet may have temporarily allowed liquid water on the martian surface.

"The decay of the martian atmosphere—which is dominated by carbon dioxide—is a component of the long-term environmental change on Mars from a climate that once allowed rivers to flow, to the cold and dry conditions of today. The minimum size of craters serves as a proxy for palaeopressure of planetary atmospheres, because thinner atmospheres permit smaller objects to reach the surface at high velocities and form craters. The Aeolis Dorsa region near Gale crater on Mars contains a high density of preserved ancient craters interbedded with river deposits and thus can provide constraints on atmospheric density at the time of fluvial activity. "

"Decay of the CO2-dominated atmosphere is an important component of long-term environmental change on Mars, but direct constraints on paleoatmospheric pressure P are few. Of particular interest is the climate that allowed rivers to flow early in Mars history, which was affected by P via direct and indirect greenhouse effects. The size of craters embedded within ancient layered sediments is a proxy for P: the smaller the minimum-sized craters that form, the thinner the past atmosphere. Here we use high-resolution orthophotos and Digital Terrain Models (DTMs) to identify ancient craters among the river deposits of Aeolis close to Gale crater, and compare their sizes to models of atmospheric filtering of impactors by thicker atmospheres. "